1. Field of the Invention
The present invention relates to a detector for detecting the flow of a first liquid in a circuit for circulating a second liquid and, in particular, to a detector for detecting the flow of a medical liquid in a circuit for circulating blood outside the body.
2. Description of the Related Art
This invention is applicable in blood treatment techniques that make use of blood circulation outside the body and in which it is necessary to inject a medical liquid into the patient (for perfusion or for substitution purposes), which techniques include, for example, hemofiltration, hemodiafiltration, and plasmapheresis.
In these treatment techniques, the medical liquid is generally inserted into the blood in a circuit for circulating the blood outside the body, thereby avoiding the need to multiply direct accesses to the vascular system of the patient. Thus, in conventional medical liquid circuits, the duct along which the medical liquid flows from a supply of medical liquid is connected at one of its ends to one of the ducts in the blood circuit, either upstream or downstream of a blood treatment device.
An accident that occurs quite frequently in blood treatment installations of the type mentioned above, is an involuntary interruption in the flow of medical liquid, e.g. due to forgetting a clamp on a duct in the medical liquid circuit or due to stopping of the pump used for causing the medical liquid to flow and for adjusting its flow rate. An accident of this kind can have serious consequences for the patient if it is not detected quickly, and two main detection means are presently in use, either singly or in combination, for the purpose of detecting that the medical liquid is not flowing.
A first detection means used in medical liquid circuits including a flow pump is constituted by means for detecting pump rotation. Such detection means provide a partial solution only to the problem posed, since they serve to detect only one of the possible causes of the medical liquid flow being stopped, namely that the pump is stopped. In particular, such means cannot detect the presence of a clamp on a duct in the medical liquid circuit.
A second detection means that is used comprises a pressure detector responsive to the pressure in a degassing chamber which is generally provided in the medical liquid circuit, either upstream or downstream of the circulation pump, assuming the circuit includes one. Such detection means are also not reliable, since whether or not the detector can detect that there is no flow of medical liquid depends on the position of a clamp on the medical liquid duct relative to the degassing chamber and to the pump. For example, if a clamp is placed on that portion of the duct which extends between the degassing chamber and the pump, then there is no significant change in the pressure in the degassing chamber, regardless of whether the degassing chamber is disposed upstream or downstream from the pump relative to the flow direction of the medical liquid.
It should also be observed that the inadequacy of these detection means cannot be mitigated by using conventional flow detectors suitable for use on a medical liquid circuit. Given the special nature of a circuit of this type, namely that it is for use only once and that it is required to contain a sterile liquid which must not be polluted under any circumstances, a flow detector suitable for use on such a circuit must not have any part that comes into contact with the liquid, and more generally must be external to the medical liquid circuit. Unfortunately, conventional flow detectors of this type, e.g. ultrasonic detectors, are unsuitable for reliably detecting the flow of a liquid that is perfectly clear and that does not convey any particles or microbubbles.